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during a flood event. An inflow and outflow structure will be constructed between the River and <br /> the mining pod once mining is within 300 feet of the river. These structures will allow for the safe <br /> exchange of flood waters between the pit and river which prevents erosion of the riverbank and <br /> pitside slope during flood events. These structures are of a design approved for use by the Mile <br /> High Flood District. Details of the inflow/outflow structure are shown on Map F-2. <br /> 6.2. Disturbed Area Runoff <br /> During all stages of mining, there is enough water storage capacity to contain the 5-year and <br /> 100-year 24-hour storm events and prevent erosion from surface water discharge. The <br /> expected rainfall from these events at the Evans Site is provided in Table G-2 below. <br /> Table G-2. Area Storm Events (from NOAA2) <br /> Event Probability <br /> 100-YR 24-HR 4.74 <br /> The peak runoff was generated from these values for the three drainage basins during all <br /> stages of mining. Pre-mine, mining, and reclamation conditions are delineated on the Drainage <br /> Map. The discharge volumes from these storm events are calculated in Appendix G-1 at the end <br /> of this exhibit. Table G-3 summarizes the runoff volumes and storage volumes for each <br /> drainage. All drainage calculations were made using the Rational Method identified in the Mile <br /> High Flood Control District. <br /> Table G-3. Drainage Calculations <br /> Draina.(Ye Basin Runoff& Destination <br /> Drainage Phase of Area Runoff 100-YR 24- Detention Discharge Flow <br /> Basin Operation (ac) Coefficient HR Runoff Capacity Rate(gpm)* <br /> (ac-ft) (ac- <br /> ft *** <br /> 1 Baseline 39.8 0.23 I 3.09 N/A 0 <br /> Mining 0.37 5.79 500+ 1000-3000** <br /> Reclamation 0.59 9.24 43 0 <br /> Reclamation 0.23 3.09 N/A 0 <br /> Alternative <br /> 2 Baseline 120 0.22 9.69 N/A 0 <br /> Mining 0.4 18.87 2000+ 1000-3000** <br /> Reclamation 0.7 36.74 156 0 <br /> Reclamation 0.21 9.25 N/A 0 <br /> Alternative <br /> *The discharge flow rate is calculated from the peak discharge of the 100-Yr 24-Hr storm event. <br /> **Discharge flow rate is variable and controlled during mining as all discharges are pumped from the pit <br /> ***Detention Capacity calculated in CAD as volume in pit or volume above water storage. <br /> 2 National Oceanic and Atmospheric Administration <br /> Evans Mining Resource G-6 Lewicki gl Associates <br /> February 2025 <br />